Due to the deepening of oil wells and gas wells (hereafter, oil wells and gas wells are collectively referred to as “oil wells”), there is a demand to enhance the strength of oil-well steel pipes. Specifically, oil-well steel pipes of 80 ksi grade (having a yield stress of 80 to 95 ksi, i.e., 551 to 655 MPa) and of 95 ksi grade (having a yield stress of 95 to 110 ksi, i.e., 655 to 758 MPa) have been widely used, and recently, oil-well steel pipes of 110 ksi grade (having a yield stress of 110 to 125 ksi, i.e., 758 to 862 MPa) and of 125 ksi grade (having a yield strength of 125 to 140 ksi, i.e., 862 to 965 MPa) have been demanded.
Deep wells are often situated in sour environments, which contain corrosive hydrogen sulfide. An oil-well steel pipe used in such a sour environment is required to have not only a high strength but also a sulfide stress cracking resistance (hereafter, referred to as SSC resistance) and a delayed fracture resistance (which are also collectively referred to as a hydrogen embrittlement resistance).
A steel with enhanced hydrogen embrittlement resistance is proposed in Japanese Patent Application Publication No. 56-5949 (Patent Literature 1) and Japanese Patent Application Publication No. 57-35622 (Patent Literature 2). The steels disclosed in these literatures contain Co, so as to enhance their hydrogen embrittlement resistance characteristics (SSC resistance, delayed fracture resistance).
Specifically, a high-tensile steel disclosed in Patent Literature 1 is produced by quenching and tempering a steel that contains a chemical composition containing C: 0.05 to 0.50%, Si: 0.10 to 0.28%, Mn: 0.10 to 2.0%, Co: 0.05 to 1.50%, Al: 0.01 to 0.10%, with the balance being Fe and unavoidable impurities, and has a proof stress of 60 kg/mm2 or more.
A high-strength steel for oil well disclosed in Patent Literature 2 is produced by quenching a steel at 880 to 980° C. and tempering at 650 to 700° C., the steel that contains a chemical composition containing C: 0.27 to 0.50%, Si: 0.08 to 0.30%, Mn: 0.90 to 1.30%, Cr: 0.5 to 0.9%, Ni: 0.03% or less, V: 0.04 to 0.11%, Nb: 0.01 to 0.10%, Mo: 0.60 to 0.80%, Al: 0.1% or less, and Co: 3% or less, with the balance being Fe and unavoidable impurities, the unavoidable impurities containing P: 0.005% or less, S: 0.003% or less.
However, for the Co-containing steels of Patent Literature 1 and Patent Literature 2, their strengths can be insufficient when their contents of C are low. Hence, in regard to steel pipes for an oil-well steel pipe put to practical use, there is no stable production of oil country tubular goods of 125 ksi grade (having a yield strength of 862 MPa or more), which has SSC resistance allowing the oil country tubular goods to endure a standard condition for a constant load test (H2S environment at 1 atm) according to the NACE (National Association of Corrosion Engineers) TM0177 Method A.
Japanese Patent Application Publication No. 2006-265657 (Patent Literature 3) proposes an oil-well steel pipe of which a C content is increased to obtain a high strength.
An oil-well steel pipe disclosed in Patent Literature 3 is produced by tempering on a low alloy steel after performing oil cooling quenching or austempering, the low alloy steel that has a chemical composition containing, in mass %, C: 0.30 to 0.60%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%, Al: 0.005 to 0.10%, Cr+Mo: 1.5 to 3.0%, where Mo is 0.5% or more, V: 0.05 to 0.3%, with the balance being Fe and impurities in which P is 0.025% or less, S is 0.01% or less, B is 0.0010% or less, and O (oxygen) is 0.01% or less, and including a steel microstructure made of a bainite single phase. Patent Literature 3 describes that the above producing method provides a steel for oil well or an oil-well steel pipe that inhibits quench cracking likely to occur in quenching a high-carbon, low-alloy steel and has an excellent SSC resistance.
Now, conventional evaluation of SSC resistance on steel materials is based on, for example, tensile tests or bending tests such as a Method A test or a Method B test regulated in NACE TM0177. These tests use unnotched test specimens and have no consideration about SSC propagation arresting characteristics. Therefore, even steel materials evaluated by these tests as having excellent SSC resistance may suffer SSC due to propagation of latent cracks in the steel.
Accompanying the deepening of oil wells and the like in recent years, steel materials for oil country tubular goods are required to have more excellent SSC resistance than conventional practice. Accordingly, to further enhance the SSC resistance, it is preferable not only to prevent SSC from occurring but also to inhibit SSC from propagating. To inhibit SSC from propagating in steel, the steel needs to be enhanced in toughness. From this viewpoint, a DCB (Double Cantilever Beam) test of Method D regulated in NACE TM0177 has been imposed. Steel materials for oil country tubular goods used in a highly corrosive environment are required to provide high fracture toughness value (hereafter, abbreviated to KISSC) in a DCB test.
However, Patent Literature 1 to Patent Literature 3 give no consideration about the fracture toughness value in the DCB test.